So I went to mpix.com as advised and I setup their ICC profiles under the custom setting as instructed by their PDF (and I have my monitor calibrated).

I then duplicated the image in photoshop and did a split screen.

On the left I had my original image and on the right I had the duplicate. To the duplicate I applied the mpix.com custom proof settings. Once I did, the duplicate image with the mpix settings looked very washed out as compared to the original.

So at this point in the workflow do I now go and edit this duplicate image so that it looks good on my screen? Will this mean that it's matched up for my monitor and their printer?

On the left I had my original image and on the right I had the duplicate. To the duplicate I applied the mpix.com custom proof settings. Once I did, the duplicate image with the mpix settings looked very washed out as compared to the original.

So at this point in the workflow do I now go and edit this duplicate image so that it looks good on my screen?

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Don't duplicate the image. Just turn on soft-proofing and, using the mpix profile, adjust the color the way you want the print to look. Use adjustment layers (curves, hue/sat, whatever) and label them as "mpix." Save that version as a jpeg to send to mpix.